This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Transit time ultrasonic flowmeters are capable of high accuracy performance over a wide range of application conditions. This has led to their adoption in applications such as custody transfer of liquid hydrocarbons. In the majority of applications, the combination of velocity, pipe diameter and viscosity are such that the flow is turbulent. Turbulent flow is characterized by the presence of turbulent vortices or ‘eddies’ that provide cross-stream mixing of the flow.
In some applications, such as the production and transportation of ‘heavy oil’, the fluid viscosity is greater than normal, with the result that the flow may be in the transitional or laminar regimes. Transitional flows typically occur in the region where Reynolds number is between 2,000 and 10,000. Laminar flows typically occur at Reynolds numbers below 2,000. In laminar conditions the flow essentially travels parallel to the axis of the conduit, with no cross-stream mixing. In the transitional flow regime the flow essentially switches back and forth between laminar and turbulent conditions.
When flow is in the laminar regime, the lack of turbulent mixing means that temperature gradients can form in the fluid. If, for example, the fluid flow entering a section of pipe is at a higher temperature than the pipe itself, then the fluid directly next to the pipe wall will be cooled to the temperature of the pipe wall, and a temperature gradient will develop between the wall and the centre of the pipe. The form of the temperature gradient will vary depending on factors such as the flow velocity, the temperature differential, the thermal conductivity of the fluid and distance along the conduit. Typically, in the applications of interest, the temperature will change rapidly in a region close to the pipe wall.
Transit time ultrasonic flowmeters operate by estimating flow velocity, and hence volumetric flowrate, by measuring the flight time of ultrasonic pulses. For applications that demand high accuracy, normally the ultrasonic transducers are installed in a housing that is integrated into a pipe spool such that the face of the housing is at an angle (typically 45°) to the pipe axis. A further aspect of flow meter design typical for high-accuracy applications, is that the transducer housing will not protrude beyond the inside wall of the conduit. As such a cavity is formed in front of the housing, and the ultrasound passes through the fluid in this cavity before traversing the cross-section of the conduit and passing through a second cavity in front of the receiving transducer. When the fluid between the faces of the two transducer housings is homogenous and isothermal, the ultrasound essentially travels in a straight path. However, when thermal gradients exist in laminar flow conditions, the fluid trapped in the cavities will take on the pipe wall temperature. As the velocity of sound is a function of temperature, the result is that the ultrasound must now undergo refraction as it travels from one transducer to the other. This means that instead of traveling along a path that is straight and constant, the path taken by the ultrasound is now a function of the process fluid, temperature and flow conditions.
Even in the case where the transducers are mounted external to the conduit, such as in so called clamp-on ultrasonic flowmeters, the presence of a thermal gradient will result in additional refraction of the ultrasonic path such that it will be different from assumptions applied in the flow meter's calculation algorithm.
Fluid flow meters are often deployed with some form of upstream flow conditioning device. In general these are deployed in order to remove non-axial components of flow velocity and/or to reshape the velocity profile across the pipe. Examples are tube bundles (FIG. 1a) and vane-type conditioners (FIG. 1b) which predominately aim to remove non-axial flow components by subdividing the flow into channels which are longer in the direction of the pipe axis than they are in cross-section, thus breaking up large vortices and increasing the tendency of the flow to travel parallel to the pipe axis.
Perforated plate flow conditioners are designed with the intent of both removing non-axial flows and reshaping the axial velocity profile. This is achieved by using perforations in a plate that divide the flow into a series of jets as illustrated in FIG. 2. The flow is redistributed as a result of the pressure differential across the plate and turbulent mixing of the jets downstream of the plate produces a flow velocity distribution that is essentially uniform and free of bulk non-axial flow components.
Tab-type flow conditioners such as the proprietary Vortab device, use tabs 1 to generate large vortices that mix the flow, destroying any bulk non-axial flow components that exist upstream and redistributing the axial velocity profile. These vortices then dissipate downstream so that the velocity field presented to the meter is improved relative to disturbed conditions that may exist upstream of the device. An example of a tab type conditioner is shown in FIGS. 3a and 3b. 
None of these devices were developed for application to laminar flow, or the particular problem of thermal gradients at the boundary. They are normally deployed in turbulent flow conditions, for the purposes described above, or sometimes for mixing. As such they are deficient in addressing the particular problem at hand. Tube bundle and vane conditioners are not designed to mix the flow or disturb the boundary layer, and hence have little impact on the thermal boundary layer as it passes through. In the case of plate and tab-type conditioners, although these can be used for mixing in turbulent flow conditions, they are ineffective at solving the problem of thermal gradients at the boundary in laminar flows. This is because (1) there are areas where the boundary layer flow can pass through relatively unaffected, and (2) in laminar flows when the boundary layer becomes separated from the wall, it tends to reattach in such a way that the thermal gradient is largely preserved.
This can be illustrated with reference to a tab-type conditioner. A conventional tab-type conditioner has a group of four tabs at each of a number of locations spaced along the axis of the conduit as illustrated in FIGS. 3a and 3b. Looking down the conduit, the tabs 1 from each group are aligned with one another as shown in FIG. 3a. Therefore, in the zones 2 between the tabs, the boundary layer at the wall can pass through undisturbed, as shown in FIG. 3a. Furthermore, when the laminar boundary layer 3 is forced off the wall by the presence of a tab, it reattaches downstream, creating a recirculation zone or dead zone 4 behind the tab. This is illustrated in FIG. 4 for a single tab in two-dimensional form. The fluid trapped in the zone behind the tab will take on the temperature of the boundary layer 3 and hence a thermal gradient will still be present in the reattached boundary layer 5.
Another related field is the mixing of two fluids or the homogenization of a single fluid in a conduit, the latter including application to temperature redistribution in heat exchangers. In laminar flow conditions, static mixers are known that are made up of arrays of planar or curved blades. These blades are combined in assemblies, with blades arranged alternatively in two or more planes, these planes typically being at 45° to the conduit axis and 90° to one another, as illustrated in FIGS. 5a and 5b. Additional planes of blades are often included in a single assembly as illustrated in FIG. 6. In a single assembly, all of the blades are parallel with respect to one another (e.g. horizontal or vertical). For more effective mixing, this type of mixer may be comprised of several sub-assemblies with the blades of one subassembly at a different angle to another subassembly as shown in FIGS. 7a and 7b. It is characteristic of these mixers that the blades extend completely across the conduit and when viewed looking down the axis of the conduit, they leave no unobstructed area for straight-through passage of laminar flow (e.g. FIG. 5a).